Field of the Invention
Aspects of the present invention generally relate to a communication apparatus, a control method for a communication apparatus, and a storage medium.
Description of the Related Art
Most of facsimile apparatuses are supposed to be directly connected to a public line network (for example, the public switched telephone network (PSTN)). Therefore, facsimile apparatuses are tightly managed in such a way as to conform to predetermined technical standards of the PSTN.
On the other hand, due to the development of information and communication equipment, communication apparatuses that perform sending and receiving of voice data using Voice over Internet Protocol (VoIP) technologies with the use of Internet Protocol (IP) networks having broadband transmission channels, such as Digital Subscriber Line (DSL) and fiber-optic communication lines, are in widespread use.
For example, in a case where a telephone set connected to a facsimile apparatus adapted to the PSTN is used to perform voice communication with the use of VoIP technologies, a voice signal output from the facsimile apparatus needs to be converted into a signal of the format conforming to the IP network. Therefore, the facsimile apparatus is connected to the IP network via, for example, a VoIP adapter, which functions as an interface for such signal conversion. The VoIP adapter, which is connected to such a facsimile apparatus, is usually called a “terminal adapter”, and includes known apparatuses of various communication formats, such as a business phone system. The main equipment of a recent business phone system is equipped with an interface used for connection of a telephone set or facsimile apparatus adapted to the PSTN, and functions as the above-mentioned terminal adapter.
Such terminal adapters, which are not directly connected to the PSTN, are, therefore, not required to meet the predetermined technical standards needed to connect to the PSTN, and are often structured according to unique specifications, such as that described below, convenient for the respective terminal adapters.
For example, the telephone exchange installed on a line is required to send out a call indicator (CI) signal and, when detecting the off-hook state of a facsimile apparatus, to stop sending out the CI signal. However, in recent years, terminal adapters that do not immediately stop sending of the CI signal have appeared in the market.
Furthermore, while facsimile apparatuses perform adjustment to have a predetermined load impedance when going off-hook, the telephone exchange installed on a line is required to supply an off-hook current corresponding to the predetermined load impedance. However, in recent years, terminal adapters of the constant-current operation type that perform limitation in such a way as to be able to supply only a constant current for the purpose of reducing operating power have appeared in the market. While the terminal adapters of the constant-current operation type are the ones configured to limit off-hook current to a minimum for the purpose of enabling battery-powered operation, the predetermined technical standards, which are essentially needed to connect to the PSTN, do not have such specified limitation.
A facsimile apparatus connected to such a terminal adapter of the constant-current operation type is configured to perform impedance adjustment when going off-hook in such a way as to obtain a constant off-hook current to which the terminal adapter limits the off-hook current. Moreover, in the above-mentioned constant-current operation type, a current difference between an off-hook current available for operation of the terminal adapter and an off-hook current insufficient for operation thereof is small. This means that the range of off-hook currents available for operation of the terminal adapter of the constant-current operation type is narrow. Thus, as a result, a facsimile apparatus connected to such a terminal adapter of the constant-current operation type needs to perform impedance adjustment in such a way as to cause the off-hook current to fall within the narrow off-hook current range.
Furthermore, as an extreme example, in a case where an off-hook current supplied from the terminal adapter is not sufficient for a load impedance for off-hook of a facsimile apparatus, the terminal adapter may sometimes not recognize a line closed state. In such a case, even when the facsimile apparatus detects a CI signal output from the terminal adapter and performs an off-hook operation, the terminal adapter would endlessly continue outputting the CI signal.
FIG. 16 is a timing chart illustrating a condition in a case where the terminal adapter does not recognize a line closed state during setting of facsimile automatic reception.
For example, in the PSTN in Japan, the line voltage during a line open state is DC 48 volts (V), the CI signal is a repetitive pattern (hereinafter referred to as a “cadence”) in which an alternating-current signal with a frequency of 16 hertz (Hz) and a voltage of 75 volts root mean square (Vrms) is alternately in an on-state for one second and in an off-state for two seconds. Here, the line condition of the facsimile apparatus during a line open state is assumed to be the same as in the PSTN.
The facsimile apparatus, when waiting in the automatic reception mode, performs an off-hook operation in response to an off-state of the CI signal, which is output from the terminal adapter, after the on-state of the CI signal is detected two times. When the facsimile apparatus performs the off-hook operation, the input impedance thereof lowers, a line current supplied from the terminal adapter flows through the facsimile apparatus, and the line voltage decreases according to the amount of the flowing line current. However, since the flowing line current is not a line current sufficient for the terminal adapter to recognize a line closed state, the terminal adapter would endlessly continue outputting the CI signal until the sending source terminates communication.
Japanese Patent Application Laid-Open No. 2012-249196 and Japanese Patent Application Laid-Open No. 2012-49827 discuss a technique in a conventional facsimile apparatus to protect circuits of the facsimile apparatus from a CI signal (a residual CI signal) that is belatedly output from the line side during a line closed state.
As mentioned above, a facsimile apparatus connected to a terminal adapter of the constant-current operation type is configured to perform impedance adjustment when going off-hook while being limited by a constant off-hook current predetermined by the terminal adapter. However, a conventional facsimile apparatus is not configured to perform impedance adjustment in consideration of such characteristics of the terminal adapter as to supply a constant off-hook current as mentioned above. While the off-hook current becomes almost equal to a current value subjected to the current limitation, the conventional facsimile apparatus is not able to determine whether the off-hook current has become a current value required for the terminal adapter to operate, in other words, whether the impedance has been adjusted to a sufficient small value. Thus, despite a constant off-hook current being supplied to the facsimile apparatus, an off-hook current required for the constant-current characteristics of the terminal adapter may sometimes not be reached, so that, in actuality, the terminal adapter may not operate.
Furthermore, the technique discussed in Japanese Patent Application Laid-Open No. 2012-249196 and Japanese Patent Application Laid-Open No. 2012-49827 is premised on that the line side normally recognizes a line closed state in response to an off-hook operation of the facsimile apparatus, and thus does not take into consideration a case where, even when the facsimile apparatus performs an off-hook operation, the line side does not recognize a line closed state. Therefore, when the line side (terminal adapter side) does not recognize a line closed state during setting of facsimile automatic reception, the conventional facsimile apparatus may be unable to perform a reception operation. Moreover, in some cases, the CI signal continuing being endlessly received may damage circuits of the facsimile apparatus.